Within and between sample comparisons of Gompertz parameters for Salmonella enteritidis and aerobic plate counts in chicken stored in air and modified atmosphere

The objective of this work was to compare between and within sample differences in aerobic plate counts (APC) and Salmonella in chicken held at abusive temperatures under air and modified atmosphere (MA). Three independent samples and three subsamples of ground raw chicken meat were inoculated with a nalidixic acid-resistant Salmonella enteritidis var. Rochester, packaged in barrier films containing air or 75% CO₂, 20% N₂, and 5% O₂, and stored at 13 and 27°C for 120 h. Samples were quantitatively enumerated for APC and S. enteritidis colony forming units (cfu/g). Gompertz function parameters for microbial growth in each sample and subsample were statistically compared in order to determine if within sample variation was different to between sample variation. At 13°C, APC organisms averaged 1.27 log units lower in the MA compared to air while Salmonella counts averaged 1.44 log units higher. At 27°C APC averaged 0.46 log units lower in the MA compared to air while Salmonella averaged 0.30 log units lower. The data fit the Gompertz function such that no systematic deviations from the curves were detected and the confidence bands were relatively narrow. Except for initial microbial load, there were no significant differences in variance of Gompertz parameters between the subsamples and the independent samples. This indicates that samples from different chickens with the same initial APC can be expected to be as similar in growth patterns and final counts as subsamples from the same chicken.